Restaurants and institutions commonly hold and serve hot foods from electrically operated countertop food warmers. Water is usually, but not necessarily, used in the wells of the warmer to act as a heat transfer medium and to improve thermal efficiency. These warmers are designed to hold 12".times.20" steam table pans or combinations of fractionally sized pans in various depths with the use of an adaptor plate formed from a sheet of metal with one or more openings. The warmers may also be used to hold a variety of round shouldered vegetable pans or inserts. The rectangular vegetable pans or inserts are supported by a lip which extends outwardly at the top of the pan and rests on the top edge of the warmer well. In addition to support, the lip acts as a loose seal to prevent the escape of large amounts of steam from the area between the bottom of the well and the bottom of the pan. Like the rectangular pans, the adaptor plates also have lips and flanges to provide support and act as a steam seal.
The arrangement commonly employed for use in restaurants and cafeterias prior to the present invention is exemplified by U.S. Pat. No. 4,284,880 which describes a metal food pan surrounded by an insulated housing that has a pair of spaced apart metal walls between which is stuffed fiberglass insulation. An electrical heating element provided in the unit is not exposed to the metal food tray but is instead mounted below a metal plate which is itself spaced a few inches below the food tray. U.S. Pat. No. 4,215,267 is similar except that the electrical heater is mounted below a heavy heat distribution plate. Heat is not transmitted efficiently to the food tray in these devices because the electric heater is in physical contact with the housing and is not facing, i.e., exposed directly to, the food tray. Moreover, the double-walled housing filled with insulation is expensive, time consuming to assemble, and presents an undesirable environmental impact. U.S. Pat. No. 5,045,672 is generally similar except that the water pan which consists of a single thickness of sheet metal will suffer from substantial heat losses, defeating a primary objective of the invention which is to provide excellent thermal efficiency while at the same time eliminating the need for an expensive insulation-filled metal housing.
U.S. Pat. No. 3,130,288 discloses a food service device which includes a deep outer pan containing a water bath and an inner tray for holding food. Both the tray and the pan are formed from transparent plastic sheet material such as a thermoplastic (Lucite or Plexiglas) or a thermosetting material. An electric heating element is placed in contact with the inside of the pan. This device has several shortcomings. First, the electric heater can cause localized overheating and damage to the material from which the pan is formed. More importantly, because both the food tray and the water bath pan are formed from plastic material, they are both heat insulators. Consequently, heat is not transmitted efficiently to the food. Finally, much infrared radiation will escape through the transparent plastic walls of the unit.
In view of these and other deficiencies of the prior art, it is one object of the invention to provide an improved food warmer for restaurants, cafeterias and the like in which heat loss is minimized and the requirement for an expensive fiberglass-filled sheet metal housing is eliminated while at the same time enabling heat to be conducted very efficiently from the electric heating element to the food within the food tray.
Another object of the invention is to provide a food warming device for cafeterias, restaurants and the like having a rigid monolithic sump vessel of a composition which provides excellent strength and impact resistance, outstanding heat insulating qualities and is not subject to stress cracking or other damage after repeated cycles of exposure to boiling water over a period of many months or years of use.
These and other more detailed and specific objects of the present invention will be apparent in view of the following description setting forth by way of example but a few of the various forms of the invention that will be apparent to those skilled in the art once the principles described herein are understood.